The primary objective of this Phase II study is to test the diuretic efficacy of single intravenous doses of CVT-124 at three dose levels in patients with stable NYHA class II and III congestive heart (CHF) assessed by changes in electrolyte excretion, urine volumes, and weight. The secondary objective of this study is to assess the safety and tolerability of a single IV dose of CVT-124 at three dose levels in patients with stable CHF.